The histone proteins are responsible for the compaction and functional organization of DMA in the nucleus. These small, evolutionary conserved proteins are host to a diverse array of post-translational modifications, each of which is typically associated with a specific DNA-templated process, such as gene regulation. We recently discovered a novel mammalian gene silencing pathway mediated by the PR-Set7 enzyme that specifically monomethylates histone H4 lysine 20 (H4K20). Our new findings indicate that this silencing pathway is targeted to the promoters of certain genes that function in growth and differentiation programs. Based on these findings, our central hypothesis is that one major role of H4K20 monomethylation is to maintain cellular identity by repressing specific sets of genes that promote growth and differentiation. The goal of this proposal is to dissect the molecular mechanisms of this fundamental gene silencing pathway with the long range goal of determining the role of this pathway in establishing and maintaining cellular identity. We recently discovered that PR-Set7 associates with a novel histone H3 lysine 9 (H3K9) methyltransferase to create a repressive monomethyl-H4K20 and H3K9 "trans-tail histone code" at gene promoters. In Aim 1 we will use established molecular and biochemical methods to identify and characterize this enzyme and determine its in vivo role in gene repression. We have also discovered the first known H4K20 monomethyl- binding protein, L(3)MBT, and determined that this interaction is essential for gene repression. In Aim 2 we will define the regions and amino acids of L(3)MBT required for binding monomethylated H4K20 and gene repression. In Aim 3 we will continue to identify additional genes regulated by this silencing pathway. These genes serve as the in vivo models to dissect apart the individual contributions of each of the components of this pathway on in vivo gene repression. Collectively, this proposal will illuminate the molecular mechanisms of a fundamental mammalian gene regulation pathway involved in critical biological processes and will likely have far-reaching and wide-spread impacts on human health and disease.